A resonator optical gyroscope is a rotation rate sensing device that includes a resonant cavity. The resonant cavity supports light waves propagating in opposite directions (without loss of generality, they are referred to in the following as clockwise (CW) and counter-clockwise (CCW) directions, respectively). When there is a non-zero rotation rate around the normal axis of the resonator, the effective optical round-trip path length for the CW and CCW lightwaves is different, leading to a resonant frequency difference between them. By measuring this resonant frequency difference, rotation rate can be determined.
A resonator fiber-optic gyroscope (RFOG) is a special kind of resonator gyroscope that uses optical fibers in the resonator. Optical fiber increases the gyro signal-to-noise (S/N) sensitivity without significantly increasing the size of the sensing loop. For measuring the resonant frequency difference, monochromatic light waves are typically sinusoidal phase/frequency modulated and coupled into the RFOG resonator in the CW and CCW directions. Fractions of light circulating inside the resonator are coupled out of the resonator and converted to electronic signals at photodetectors. The electrical signals are demodulated at the corresponding modulation frequencies and used to servo the input light frequencies to the resonance frequencies of the CW and CCW cavity.
There are many methods to phase/frequency modulate light waves for an RFOG. One common approach is to use a lithium niobate phase modulator after the laser. Another approach is to modulate the injection current of a semiconductor laser. If the later method is used, the modulation of the injection current not only modulates the laser light frequency but also the light intensity. The resulting intensity modulation is at the same frequency as the laser frequency modulation.
If a phase modulator is used for phase modulation, along the optical path before and after the RFOG's phase modulator there can be polarization cross-coupling points due to imperfect fiber splices or polarization axis mismatch between the modulator waveguide and its pigtail fibers. Most of the optical power propagates in the optical path whose polarization state is aligned with the pass-axis of the modulator, as intended. But a small amount of cross-coupled optical power propagates in the optical path whose polarization state is orthogonal to the path's access of the modulator. At a cross-coupling point after the phase modulator, the interferences between the light waves propagating along the two orthogonal polarization axis of the modulator causes intensity modulation at the phase modulation frequency.
Intensity modulation leads to a non-zero photo detector output signal at the resonance center frequency. The non-zero photo detector output at resonance frequency causes the servo to move the laser frequency off the resonance frequency to eliminate the error. If both the CW and CCW laser frequencies are moved off resonance by the same amount and in the same direction (positive or negative phase), then the rotation sensing errors could cancel. However, it is unlikely that the intensity modulation in both beams is of the same magnitude and in the same direction.